This invention is in the field of electrochemical devices, and relates generally to electrolytes for extending the operating temperature range of lithium-ion electrochemical cells. Electrolyte compositions, electrochemical cells employing the electrolyte compositions, and methods of making and using the electrochemical cells are provided.
A number of technical barriers associated with the development of Li-ion rechargeable batteries have been identified, including their narrow operating temperature range, limited life, and poor abuse tolerance. For this reason, there is an interest in the development of advanced electrolytes which will improve the performance of batteries over a wide range of temperatures (−30 to +60° C.) and lead to long life characteristics (5,000 cycles over a 10-year life span). There is also interest in improving the high voltage stability of electrolyte systems to enable the operation of up to 5V with high specific energy cathode materials.
A number of future NASA missions and terrestrial applications, such as plug-in hybrid electric vehicles (PHEVs), require rechargeable batteries that can operate over a wide temperature range (−60 to +60° C.) and provide good life characteristics. For example, future NASA missions aimed at exploring Mars and the outer planets will require rechargeable batteries that can operate at low temperatures to satisfy the requirements of various machinery, including landers, rovers, and penetrators. Currently, state-of-the-art lithium-ion systems demonstrate operability over a temperature range from −30° C. to +40° C.; however, the rate capability at lower temperatures is poor due to poor electrolyte conductivity, poor lithium intercalation kinetics over the electrode surface layers, and poor ionic diffusion in the electrode bulk. In addition, the low temperature performance deteriorates rapidly once the cell has been exposed to high temperatures. However, improved rate capability of lithium-ion systems is desired at these very low temperatures (−30° to −70° C.), as well as good tolerance to warm temperatures.
Several factors can influence the low temperature performance of lithium-ion cells, including: (a) lithium ion mobility in the electrolyte solution (electrolyte conductivity), (b) interfacial characteristics (permittivity of ions through the solid electrolyte interphase layer, or “SEI” layer), (c) inherent properties of the electroactive materials (such as diffusion characteristics), and (d) cell design properties (such as electrode thickness, separator porosity, separator wetting properties, etc.). Of these parameters, the electrolyte-induced properties can be the most dominant, in that sufficient conductivity is a necessary condition for good performance at low temperatures. In designing electrolytes with high conductivity at low temperatures, it is desirable that the solvents possess a combination of several properties, such as: high dielectric constant, low viscosity, adequate coordination behavior, as well as appropriate liquid ranges and salt solubilities in the medium.
Reported all-carbonate based electrolyte compositions for lithium ion cells include an electrolyte formulation comprising LiPF6 dissolved in a ternary, equi-proportion mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) (1:1:1 vol %); this electrolyte was demonstrated to provide long life over a wide temperature range (−30° to +40° C.) (Smart et al., NASA Technical Report (NTR) NPO-20407 (Jan. 14, 1998) and U.S. Pat. No. 6,492,064). Further improvement of the low temperature performance (i.e., below −30° C.), was reported with a quaternary electrolyte formulation comprising 1.0 M LiPF6 EC+DEC+DMC+EMC (1:1:1:2 v/v) (Smart et al., NTR NPO-20605 (Nov. 5, 1998)). Subsequent development led to the identification of a number of low EC-content ternary and quaternary solvent blend electrolytes, which have enabled excellent performance down to −50° C. (Smart et al., NTR NPO-30226 (Apr. 5, 2001).
Low-melting, low-viscosity co-solvents have also been included in electrolyte mixtures. Smart et al., NTR NPO-19983 (Jul. 3, 1996) and NTR NPO-20601 (Oct. 28, 1998 have described the use of low viscosity and low melting point ester-based co-solvents, including methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), ethyl propionate (EP), and ethyl butyrate (EB), in multi-component electrolyte formulations The work reported in NTR NPO-19983 involved the following types of solutions: 0.50 M LiPF6 in EC+DEC+methyl acetate (15:35:50 v/v/%), 0.50 M LiPF6 in EC+DEC+methyl formate (15:35:50 v/v/%), 0.50 M LiPF6 in PC+DEC+methyl acetate (15:35:50 v/v/%), 0.50 M LiPF6 in PC+DEC+methyl formate (15:35:50 v/v/%), The work reported in NTR NPO-20601 involved the following types of solutions: 1.00 M LiPF6 in EC+DEC+DMC+ester (1:1:1:1 v/v %), where the ester=MA, EA, EP, or EB. Electrolytes were also investigated which incorporate large proportions of ester co-solvents (up to 80% by volume) which have been demonstrated to have excellent performance at very low temperatures, in a number of systems (M. C. Smart, B. V. Ratnakumar, A. Behar, L. D. Whitcanack, J.-S. Yu, M. Alamgir, “Gel Polymer Electrolyte Lithium-Ion Cells with Improved Low Temperature Performance”, J. Power Sources, 165 (2), 535-543 (2007) and NPO-41097 (May 14, 2007). Smart et al., NTR NPO-41097 (May 14, 2007) have reported multi-component electrolytes of the following composition: 1.0 M LiPF6 in ethylene carbonate (EC)+ethyl methyl carbonate (EMC)+X (1:1:8 v/v %) (where X=methyl butyrate (MB), ethyl butyrate (EB), methyl propionate (MP), and ethyl valerate (EV)). The performance of this latter group of electrolytes enabled performance down to very low temperatures (i.e., −50 to −70° C.). Smart et al., J. Electrochem. Soc., 149(4), A361-A370 (2002) have reported that the higher molecular weight esters (e.g., ethyl propionate and ethyl butyrate) resulted in both improved low temperature performance and good stability at ambient temperatures. Excellent performance was obtained down to −40° C. with electrolytes comprising the following formulations: (a) 1.0 M LiPF6 EC+DEC+DMC+ethyl butyrate (EB) (1:1:1:1 v/v %) and (b) 1.0 M LiPF6 EC+DEC+DMC+ethyl proprionate (EP) (1:1:1:1 v/v %). In contrast, although electrolytes containing methyl acetate and ethyl acetate (low molecular weight esters) were shown to result in high conductivity at low temperatures and good cell performance at low temperature initially, their high reactivity toward the anode led to continued cell degradation and poor long term performance.
Another group of electrolytes was developed in which the EC-content was fixed at 20% and the ester co-solvent at 20%, A number of ester co-solvents, namely methyl propionate (MP), ethyl propionate (EP), methyl butyrate (MB), ethyl butyrate (EB), propyl butyrate (PB), and butyl butyrate (BB), were included in multi-component electrolytes of the following composition: 1.0 M LiPF6 in ethylene carbonate (EC)+ethyl methyl carbonate (EMC)+X (20:60:20 v/v %) [where X=ester co-solvent] (Smart et al., NTR NPO-44974 (Mar. 9, 2007)). Other compositions reported include 1.20M LiPF6 in EC+EMC+MP (20:20:60 v/v %) and 1.20M LiPF6 in EC+EMC+EB (20:20:60 v/v %), which were demonstrated to operate well over a wide temperature range in MCMB-LiNiCoAlO2 and Li4Ti5O12—LiNiCoAlO2 prototype cells. (Smart et al., NTR NPO-46976, Mar. 13, 2009) In other more recent work, methyl butyrate-based electrolytes were demonstrated to have good performance in 2.2Ah LiFePO4-based cells, most notably excellent power capability at low temperatures (i.e., −20° C. to −40° C.) (Smart et al., NTR NPO-46180 (May 2, 2008)).
Other ester-containing electrolyte compositions include those reported by A. Ohta, H. Koshina, H. Okuno, and H. Murai, J. Power Sources, 54 (1), 6-10, 1995): a) 1.5 M LiPF6 in EC+DEC+MA (1:2:2), b) 1.5 M LiPF6 in EC+DEC+MP (1:2:2), and c) 1.5 M LiPF6 in EC+DEC+EP (1:2:2). Although promising performance was reported, the incorporation of a large proportion of diethyl carbonate (DEC) is not preferred due to the undesirable effects that this solvent has upon the surface films of carbon anodes. Electrolytes containing ethyl acetate (EA) and methyl butyrate (MB) have also been reported (Herreyre et al., J. Power Sources, 97-98, 576 (2001) and U.S. Pat. No. 6,399,255). More specifically, the following electrolyte formulations were reported: a) 1.0 M LiPF6 in EC+DMC+MA, b) 1.0 M LiPF6 in EC+DMC+MB, c) 1.0 M LiPF6 in EC+PC+MB and d) 1.0 M LiPF6 in EC+DMC+EA. Good low temperature performance with the methyl butyrate-based electrolyte was reported. Other researchers (Shiao et al., J. Power Sources, 87, 167-173 (2000)) have investigated the use of methyl acetate and ethyl acetate in ternary mixtures with and without blending with toluene in an attempt to obtain improved performance to temperatures as low as −50° C. Other reports (Sazhin et al., J. Power Sources, 87, 112-117 (2000)) have involved the investigation of the performance of a number of electrolyte formulations at low temperatures, including the following: a) 1.0 M LiPF6 in EC+EMC+EA (30:30:40), b) 1.0 M LiPF6 in EC+DMC+MA (30:35:35), c) 1.0 M LiPF6 in EC+DEC+EP (30:35:35), and d) 1.0 M LiPF6 in EC+EMC+EP (30:30:40). Although good performance was demonstrated at −20° C., the performance attributes at temperatures below −20° C. were not investigated.
Electrolyte additives have also been included in electrolyte compositions. Vinylene carbonate (VC) has been reported to be an effective additive in improving the high temperature cycle life and storage characteristics (G. G. Botte, R. E. White, and Z. Zhang, J. Power Sources, 97-98, 570 (2001); C. Jehoulet, P. Biensan, J. M. Bodet, M. Broussely, C. Moteau, C. Tessier-Lescourret, Proc. Electrochem. Soc. 97-18 (Batteries for Portable Electric Vehicles), The Electrochemical Society Inc., Pennington, N.J. (1997), pp. 974-985; D. Aurbach, K. Gamolsky, B. Markovsky, Y. Gofer, M. Schmidt, and U. Heider, Electrochim. Acta, 47 (9), 1423-1439 (2002)). It is generally held that VC sacrificially polymerizes on the electrode surfaces, producing protective films preventing further electrolyte reaction at the interface. Although the bulk of the studies have focused upon its effect during the formation process upon the SEI of the carbon electrode, it is acknowledged that it influences the nature of the films on the cathode also (M. C. Smart, B. L. Lucht, and B. V. Ratnakumar, “Electrochemical characteristics of MCMB and LiNixCo1-xO2 electrodes from cells containing electrolytes with stabilizing additives and exposed to high temperature”, J. Electrochem. Soc. 155, A557 (2008); M. Fujimoto, M. Takahashi, K. Nishio (Sanyo), U.S. Pat. No. 5,352,548, Oct. 4, 1994).
Amine and coworkers have also described electrolyte formulations based on the use of vinyl ethylene carbonate with propylene carbonate-based electrolytes and demonstrated their resilience to temperatures as high as 50° C. (M. C. Smart, B. V. Ratnakumar, K. Chin, W. West, and S. Surampudi, “The Effect of Electrolyte Additives Upon the Kinetics of Lithium Intercalation/De-Intercalation at Low Temperatures” Ext. Abst. 202nd Electrochemical Society Meeting, Salt Lake City, Utah, October 20-25, 2002 (Abstract #183); J. M Vollmer, L. A. Curtiss, D. R. Vissers, and K. Amine, J. Electrochem. Soc., 151 (1), A178-A183 (2004)).
Lewis base electrolyte additives namely dimethyl acetamide (DMAC) and N-methylpyrollidone (NMP) have been investigated as stabilizing agents (C.-H., Chen, Y. E. Hyung, D. R. Vissers, and K. Amine., US Patent Application, 20030157413 (Aug. 21, 2003); C. L. Campion, W. Li, W. E. Euler, B. L. Lucht, B. Ravdel, J. DiCarlo, R. Gitzendanner, and K. M. Abraham, Electrochem. Solid-State Lett., 7, A194 (2004); C. L. Campion, W. Li and B. L. Lucht, J. Electrochem. Soc., 152, A2327 (2005); W. Li, C. L. Campion, B. L. Lucht, B. Ravdel, J. DiCarlo and K. M. Abraham, J. Electrochem. Soc., 152, A1361 (2005)). Good performance has been demonstrated for 1.0 M LiPF6 EC+DEC+DMC (1:1:1 v/v %) solutions with these additives after being subjected to high temperature storage (M. C. Smart, B. L. Lucht, and B. V. Ratnakumar, “The use of electrolyte additives to improve the high temperature resilience of Li-ion cells”, NTR NPO-44805 (Jan. 16, 2007); M. C. Smart, B. L. Lucht, and B. V. Ratnakumar, “Electrochemical characteristics of MCMB and LiNixCo1-xO2 electrodes from cells containing electrolytes with stabilizing additives and exposed to high temperature”, J. Electrochem. Soc. 155, A557 (2008)).
Mono-fluoroethylene carbonate has also been investigated in electrolyte solutions comprising FEC+EC+PC (1:3.5:3.5) primarily to prevent the exfoliation of graphite anode electrodes when used in the presence of propylene carbonate (PC) (R. McMillan, H. Slegr, Z. X. Shu, and W. Wang, J. Power Sources, 81-82, 20-26 (1999)). In a similar type of study, FEC was added to LiClO4 dissolved in PC and the lithium deposition characteristics were investigated ((R. Mogi, M. Inaba, S.-K. Jeong, Y. Iriyama, T. Abe, and Z. Ogumi, J. Electrochem. Soc., 149 (2), A1578-A1583 (2002)). FEC has also been studied in 1.30M LiPF6 solutions of EC+DEC (30:70) to improve the efficiency of Li/Si thin-film cells (N.-S. Choi, K. H. Yew, K. Y. Lee, M. Sung, H. Kim, S.-S. Kim, J. Power Sources, 161, 1254-1259 (2006)).